The field of this invention is the measurement of the index of refraction of liquids. These instruments, known as refractometers, are of 4 main types: traditional handheld refractometers, digital handheld refractometers, laboratory or Abbe refractometers, and inline process refractometers. Although refractometers are typically used for measuring the refractive index of liquids, they can be used to measure gases and solids for example the Rayleigh Refractometer used (typically) for measuring the refractive indices of gases. Refractometers may be calibrated in other engineering units such as specific gravity for example because of the predictable relationship between refractive index and specific gravity in many liquid solutions.
Traditional hand held refractometers rely on the critical angle principle. When the critical angle of incident light on an interface is exceeded total reflection occurs. The refracted angle of incident light is a function of the angle of incidence and the index of refraction of the liquid to be measured and related through Snell's Law.
Handheld refractometers use a small quantity of liquid that is placed between a prism and a small cover plate. Light traveling through the sample is either passed through to the reticle or totally internally reflected. The net effect is that a shadow line forms between the illuminated area and the dark area. It is where this shadow line crosses the scale that a reading is taken. The prism projects a shadow line onto a small glass reticle inside the instrument. The reticle is then viewed by the user through a magnifying eyepiece. Certain digital versions of this technique rely on an array of photodetectors to measure the position of the shadow line.
The necessity of placing a small quantity of liquid inside the instrument is an impediment to convenient, rapid and safe measurement of liquids that in some cases may be hazardous such as acids. It is clear that an instrument that can make the refractive index measurement by direct immersion in the container and is impervious to corrosive fluids would be both a considerable convenience and a safety measure and thus a need exists for such a device.
In the Abbe' refractometer the liquid sample is sandwiched into a thin layer between an illuminating prism and a refracting prism. Abbe' refractometers are most easily used for measuring the index of solids.
Process control refractometers are designed to monitor the refractive index under continuous flow conditions such as oil refining and various chemical processes. These refractometers rely on conventional fiber optics using the liquid as the cladding as described above and suffer from a number of shortcomings including susceptibility to corrosive liquids.